You couldn't look anywhere on Facebook without seeing it: friends, celebrities and complete strangers dumping buckets of ice water to raise awareness of ALS, a neurodegenerative illness also known as Lou Gehrig's disease.

The 2014 Ice Bucket Challenge ended up raising more than $115 million for ALS research and reached an unprecedented bar for a charity social media campaign — unprecedented and inimitable.

"It was like catching lightning in a bottle," says Ginny Simmons, who advises advocacy groups and nonprofits on digital strategy at FitzGibbon Media and has been involved in digital fundraising for a decade.

This week, the Web phenomenon of #GivingTuesday has launched us into the fundraising peak of 2015 (aka the tail end of the tax year) and with it, you're bound to see some social media fundraising call-outs. But it turns out that while Facebook and other social media sites can be good at putting issues on your radar, they are pretty ineffective at getting people to click away and actually donate.

"It's useful because people are seeing your issue," says Michael Ward, a principal at strategy firm M+R that publishes the Benchmark Study, a nonprofit industry guide to online fundraising and advocacy. "But then to actually get them to divert that knowledge into a donation, it really takes other channels, such as email marketing or even direct marketing to close that loop."

Adobe this week published a review of data the company collected last year from 43 million visits to websites of about two dozen charities and the resulting clicks on their virtual donation buttons. It found that three-quarters of visitors to charity websites arrived there either by doing a Web search or by directly typing in the URL. Only 3 percent of referrals came from social media.

To be sure, even the most successful social media campaigns (like the Ice Bucket Challenge) may not necessarily count as a social media referral in the Adobe Digital Index if people donated after googling "ALS Association" instead of following a link directly from the group's Facebook page.

But the Adobe findings also echoed the results of a Red Cross survey also done in 2014, which found that online solicitations and engagements helped sway people to donate; however, people didn't report them to be as motivational as in-person requests or emails and direct mail, which remain the bread and butter of fundraising.

The issue strikes a chord with the digital fundraising experts I interviewed. They say charities, advocacy groups and political campaigns are naturally captivated by the promise of the booming social networks, but the payout realities are far more complicated.

"Social is something that everyone keeps trying," says Simmons, "but in terms of a fundraising success, it's not an easy straight line."

The reason is pretty simple: When people are scrolling through posts, say, on Facebook, it's incredibly rare for them to decide to click away to some outside website — let alone an outside website that's asking for their credit card information.

"It's an all-inclusive environment, kind of like a resort," says Tamara Gaffney, principal analyst with Adobe Digital Index. "When you walk in the door and everything is paid for, why would you want to walk out and get dinner at some other restaurant?"

In fact, it's a challenge faced by the commercial world: Gaffney says Adobe's analysis found that only 2 percent of referrals to shopping websites come from social media. Truth is, all of it is still a pretty new frontier, so both the companies and the charities are feeling their way through, finding some approaches that do work and others that don't.

One is crowdfunding and what's known as peer-to-peer fundraising. Those are the kind of financial appeals that come from friends and other humans instead of organizations and typically ask for small sums of money — and they tend to connect with people on a personal level.

Similarly, Simmons said people posting about donations can go a long way: "The times when you do see social play a role with fundraising is when people want to make a statement about their fundraising."